The present invention relates to a tapping screw used for a member produced from a soft material.
A tapping screw has been known to be used for a member produced from a soft material having low rigidity such as synthetic resins. The tapping screw having an included angle of 40 to 50xc2x0 and longitudinal grooves formed on the threaded portion is disclosed in Japanese Utility Model Publication No. 4570/1980 by the same applicant of the present invention. This tapping screw disclosed is directed to achieving reduction in driving torque and improvement of holding strength under the interaction between the included angle and the longitudinal grooves on the threaded portion. It has long been utilized as a tapping screw for automobile application.
However, the tapping screw for a soft material involves a problem that it has an insufficient stripping torque as compared to a driving torque, wherein the former means the maximum torque limit without collapsing of an included thread formed in a material such that the screw becomes incapable of tightening, and the latter means the torque necessary for driving the tapping screw into the material. This problem results in a narrow tolerance of a tightening torque and also requires strict torque control so as to avoid such incapability of the screw in tightening thereof, leading to poor workability.
A tapping screw to be driven well into aerated concrete is disclosed in Japanese Patent Publication No. 52083/1994. However, this tapping screw cannot exhibit the desired performance of a sufficient stripping torque as compared to a driving torque.
The present invention is directed to solve the problems described above and to provide a tapping screw for a member produced from a soft material. The tapping screw of the present invention has a sufficiently large stripping torque as compared to a driving torque and an increased tolerance of a tightening torque. Here, the soft material includes a plate member produced from a soft material, such as a synthetic resin and a composite material thereof, as well as, aluminum, magnesium and alloys thereof.
The present invention relates to a tapping screw comprising threads each having a leading flank (hereinafter referred to as a first flank which faces in a driving direction of the screw) and a following flank at the opposite side (hereinafter referred to as a second flank which faces toward a head of the screw), wherein the former is designed to be steeper than the latter; an R being imparted to a crest of each thread and to the second flank side; an R being imparted also to each flank at a portion contiguous to a root thereof; and the former R and the latter R being designed to connect smoothly to each other at the root.
Here, each thread preferably has a leading flank angle (xcex81) of 0 to 10xc2x0, a following flank angle (xcex82) of 20 to 40xc2x0 and an included angle (xcex81+xcex82) of 20 to 50xc2x0.
Further, the tapping screw may have longitudinal grooves on its threaded portion.
The expression xe2x80x9can R being imparted to . . . xe2x80x9d in the present specification means to form each thread such that it has an arcuate cross section. More specifically, at the crest, it means to cut off the crest to form a convex arcuate section, while at the portion of the flank contiguous to the root thereof, it means to form a concave arcuate section in the first flank side to recede inwardly from the first flank (a cross-sectional profile of which is linear) of the conventional tapping screw, and it means to form an arcuate section in the second flank side to project roundly from the second flank (a cross-sectional profile of which is linear) of the conventional tapping screw.
This constitution can achieve both reduction in a driving torque and increase in a stripping torque, thus allow to increase the tolerance of a tightening torque.